The use of radiation therapy to treat cancer is well known. Typically, radiation therapy involves directing a beam of high energy proton, photon, ion, or electron radiation (“therapeutic radiation”) into a target or target volume (e.g., a tumor or lesion) in a patient.
Before a patient is treated with radiation, a treatment plan specific to that patient is developed. The plan defines various aspects of the therapy using simulations and optimizations based on past experiences. In general, the purpose of the treatment plan is to deliver sufficient radiation to the target while minimizing exposure of surrounding normal, healthy tissue to the radiation.
The planner's goal is to find a solution that is optimal with respect to multiple clinical goals that may be contradictory in the sense that an improvement toward one goal may have a detrimental effect on reaching another goal. For example, a treatment plan that spares the liver from receiving a dose of radiation may result in the stomach receiving too much radiation. These types of tradeoffs lead to an iterative process in which the planner creates different plans to find the one plan that is best suited to achieving the desired outcome.
A recent radiobiology study has demonstrated the effectiveness of delivering an entire, relatively high therapeutic radiation dose to a target within a single, short period of time. This type of treatment is referred to generally herein as FLASH radiation therapy (FLASH RT). Evidence to date suggests that FLASH RT advantageously spares normal, healthy tissue from damage when that tissue is exposed to only a single irradiation for only a very short period of time. FLASH RT thus introduces important constraints that are not considered in or achieved with conventional radiation treatment planning.
Typically for radiation therapy treatment, a patient first receives a CT (computed tomography) scan used to simulate the patient's treatment. A simulated treatment plan defines beam orientations and corresponding particle fluences to generate a 3D (three-dimensional) dose distribution that best achieves the physician's prescription and/or intent. Once the treatment plan has been defined, treatment can commence. It is noted that treatment uncertainties result from differences in the patient appearance at each treatment fraction compared to the CT simulation from which the treatment plan was derived. In addition, organ motion related to gross patient movement, breathing, heart function, and variable organ filling further compounds the treatment uncertainty. Various techniques are currently employed to manage organ motion in order to minimize the difference between the planned and delivered dose to the patient, including: breath holding, treatment gating, or abdominal compression. Each of these techniques has associated benefits and drawbacks, but all are designed to manage motion when treatment delivery time is over several minutes and may last as long as 60 minutes.
For example, one of the disadvantages of breath holding is that many patients do not have lung function to hold their breath for more than a few seconds; therefore, precluding them from holding their breath for the duration of an entire treatment field. It is noted that one of the disadvantages associated with treatment gating is that it requires continuous monitoring of the patient during relatively lengthy treatments, and turning the treatment beam off whenever the target volume moves outside of a predetermined volume of interest. Furthermore, treatment gating may increase the treatment time considerably, because the treatment beam may be held off for large periods of the breathing cycle. Note that abdominal compression is often poorly tolerated by most patients, as it places patients in a great deal of discomfort and can limit critical functions associated with normal organ motion, such as breathing or bowel motion.